The present invention relates to heavy oil fluid coking involving the circulation of coke through a fluidized bed coke burner for developing heat to be used in a fluidized bed coker. The invention has to do with reducing sulphur gaseous emissions from the burner.
Fluid coking is a commercially practiced process applied to heavy oil, such as bitumen, to produce lighter fractions.
The process is illustrated in FIG. 1. It involves a fluidized bed coker reactor working in tandem with a fluidized bed coke burner. In the reactor, incoming feed oil contacts a fluidized bed of hot coke particles and heat is transferred from the coke particles to the oil. The reactor is conventionally operated at a temperature of about 530xc2x0 C. Hot coke entering the reactor is conventionally at a temperature of 645xc2x0 C. to supply the heat requirement of the coker. xe2x80x9cColdxe2x80x9d coke is continuously removed from the reactor and returned to the burner. The cold coke leaving the reactor is at a temperature of about 530xc2x0 C. In the burner, the cold coke is partially combusted with air, to produce hot coke. Part of the hot coke is recycled to the reactor to provide the heat required. The balance of the hot coke is removed from the burner as product coke. The burner is conventionally operated at a temperature of 645xc2x0 C. The burner temperature is controlled by controlling the addition of air.
As mentioned, the combustion of coke in the burner is only partial in nature. On entering the burner, part of the coke particle is burned and releases volatiles. These volatiles support the combustion that provides the heat required by the reactor. The burner produces product gas which comprises fuel gas, H2S, SO2, COS and coke fines. This product gas is burned in a boiler. A flue gas leaves the boiler and is emitted to atmosphere through a stack. The flue gas contains SO2.
It is the purpose of the present invention to reduce the sulphur compound content in the burner product gas and thus in the stack flue gas.
The present invention is based on the results of an experimental program conducted to determine the effect of coke burner operating conditions on product gas composition, specifically with respect to sulphur gas production.
The following discoveries were made in the course of this program:
It was found that the volatiles, represented by CH4, were produced by coke undergoing combustion at a lower temperature than the sulphur compounds, represented by H2S. More particularly, the release of CH4 commenced at a temperature of about 380 xc2x0C. and reached a maximum rate at about 570xc2x0 C., whereas the release of H2S commenced at about 500xc2x0 C. and reached a maximum rate at about 650xc2x0 C.;
It was further found that the profile for H2S evolution at increasing temperatures took the form of a parabolic curve having steeply rising and descending legs; and
It was further found that there was very little diminution in the size of the coke particles in the course of pyrolysis in the burner.
From these observations we concluded:
That volatile gases are produced from a thin outer skin portion of the coke particle and it is these gases that combust in the burner and produce most of the required heat;
That since these volatile gases are produced at a significantly lower temperature than the sulphur-containing gases, one could reduce burner temperature and thereby reduce sulphur gas emissions, without significantly affecting the capacity of the burner to supply the heat needs of the coker;
But one would need to increase the coke circulation rate, as the temperature of the hot coke leaving the burner would now be less, in order to prevent bogging and meet the heat need of the coker
As a result of acquiring these understandings, a process was outlined involving:
maintaining the burner temperature in the range of about 550xc2x0 C.-630xc2x0 C.; and
maintaining the coke circulation rate sufficient to meet the heat requirements of the coker, for example in the range 75 tons/min to 115 tons/min, particularly preferably about 90 tons/min, at an oil throughput of 110 kB/d to the coker.
The process was tested in a plant circuit consisting of two identical cokers. The burner temperature and coke circulation rate were changed from the conventional operating conditions as follows:
The SO2 discharge at the stack was reduced from 230 tonnes/day to 180 tonnes/day.